Method of making optical fiber having depressed index core region

ABSTRACT

Disclosed is a method of making an optical fiber preform having at least one annular region of depressed refractive index. A tube of silica doped with fluorine and/or boron is overclad with silica soot. A core rod is inserted into the overclad tube and the resultant assembly is heated while chlorine flows between the tube and the core rod to clean the adjacent surfaces. When the soot sinters, the tube collapses onto and fuses to the rod. The resultant tubular structure is formed into an optical fiber which exhibits low attenuation as a result of the low seed count at the interface between the inner core and the region that is doped with fluorine and/or boron.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of U.S. patent application Ser. No.09/256,248 filed on Feb. 23, 1999, which is a continuation of U.S.patent application Ser. No. 08/795,687 filed on Feb. 2, 1997, which is acontinuation-in-part of U.S. patent application Ser. No. 08/359,392filed on Dec. 20, 1994, the contents of which are relied upon andincorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. §120 is hereby claimed.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a method of making a glass article byfusing a rod and tube such that substantially no seeds are formed at theinterface between them. The method of this invention is useful formaking low loss optical fibers, especially those fibers in which thecore includes an annular region of depressed refractive index relativeto silica.

[0003] Optical fibers having refractive index profiles such asW-profiles, segmented core profiles, and the like possess desirabledispersion characteristics. See U.S. Pat. Nos. 4,715,679 and 5,031,131for teachings of various kinds of dispersion modified optical fibers.Fibers having these kinds of refractive index profiles have often beenmade by chemical vapor deposition (CVD) processes such as plasma CVDprocesses that are capable of forming single-mode fibers the cores ofwhich include layers of different refractive indices (see FIGS. 7 and 8,for example). Such processes produce relatively small preforms. It isadvantageous to form dispersion modified optical fiber preforms byoutside vapor deposition (OVD) processes which produce relatively largepreforms or draw blanks in order to decrease the cost of making thefiber.

[0004] A typical OVD process for forming such fibers is disclosed inU.S. Pat. No. 4,629,485. In accordance with that patent, agermania-doped silica rod is formed and stretched to decrease itsdiameter. A piece of the rod is used as a mandrel upon which pure silicaglass particles or soot is deposited. The resultant composite structureis heated in a consolidation (drying and sintering) furnace throughwhich a fluorine-containing gas flows. The soot is therefore doped withfluorine and sinters on the rod. One or more additional layers of glassare formed on the outer surface of the fluorine-doped silica layer toform a blank from which a fiber can be drawn.

[0005] When soot is sintered in accordance with the aforementionedmethod, whereby fluorine is supplied to the porous preform solely by wayof the fluorine-containing muffle gas, the fluorine concentration (asmeasured by the Δ of the fluorine-containing layer) is not sufficient toprovide certain desirable optical characteristics. The typical fluorineconcentration achieved with muffle gas doping provides a −0.4%Δ whenSiF₄ is the fluorine-containing constituent. The maximum delta value forSiF₄ produced by the above-described process is −0.5%Δ.

[0006] One aspect of the invention concerns a method of making anoptical fiber preform an annular region of which consists of silicadoped with a sufficient amount of fluorine that the delta value of theannular region with respect to silica is more negative than −0.5%Δ.

[0007] As used herein, the term Δ_(a−b), the relative refractive indexdifference between two materials with refractive indices n_(a) andn_(b), is defined as

Δ_(a−b)=(n _(a) ² −n _(b) ²)/(2n _(a) ²)  (1)

[0008] For simplicity of expression, Δ is often expressed in percent,i.e. one hundred times Δ. In this discussion, n_(a) is the refractiveindex of the fluorine-doped glass and n_(b) is the refractive index ofsilica.

[0009] Another aspect of the invention concerns the collapse of a tubeof fluorine-doped and/or boron-doped glass onto a rod of core glass suchthat during the resultant fusion of the interface between those twomembers, substantially no seeds are formed.

[0010] When a fluorine-doped silica tube is collapsed onto agermania-doped silica rod, the resultant interface between those twomembers has heretofore contained many seeds, and much of the resultantpreform or blank produces unusable optical fiber. Such seed formation isless prevalent when members formed of other glass compositions such as agermania-doped silica rod and a pure silica tube are fused to form apreform.

[0011] U.S. Pat. No. 4,668,263 discloses a method for collapsing asilica tube having a fluorine-doped inner layer onto the surface of asilica rod. In accordance with that patent the collapse step isaccomplished by rotating the tube and heating it with the flame from alongitudinally travelling burner. That technique could not be employedto make dispersion modified fiber designs of the type that utilize theentire fluorine-doped tube, including the outer surface, as part of thecore region or light propagating region of the fiber. The reason forthis is that, since the flame wets the glass, i.e. introduces hydroxylcontamination, the resultant fiber would be rendered unsuitable foroperation at wavelengths where attenuation due to hydroxyl ions islarge. A further disadvantage of this method concerns the temperature ofthe flame, which is not lower than 1900° C. At such high temperatures,control of the process becomes difficult. The axis of the preform canbecome non-linear or bowed. If the core rod is a soft glass such as agermania-doped glass, the rod can become softer than the tube; this canresult in an out-of-round core or a core that is not concentric with theouter surface of the resultant fiber.

[0012] U.S. Pat. No. 4,846,867 discloses a method for collapsing afluorine-doped silica tube onto the surface of a silica rod. Prior tothe tube collapse step, a gas phase etchant is flowed through the gapbetween the rod and tube while the tube is heated by a flame. In thespecific examples, wherein SF₄ is the etchant, a gaseous mixture of SF₄,Cl₂ and oxygen (ratio 1:1:6 by volume) is introduced through a gapbetween the rod and the tube. Such a gaseous mixture removes glass fromthe treated surfaces of the rod and tube, thus forming new surfaces atthe rod/tube interface. The chlorine is present in an amount sufficientto remove water generated by the fluorine-containing etchant. The outersurface of the resultant preform is thereafter coated with silica sootparticles that are dried, doped with fluorine and then sintered to forma blank from which an optical fiber is drawn. The flame that wasdirected onto the tube during the gas phase etching step introduceswater into the outer surface of the tube. The attenuation of the fiberresulting from that water is high. The attenuation at 1380 nm for oneexample is 30 dB/km which is attributed to contact of the oxyhydrogenflame with the preform.

SUMMARY OF THE INVENTION

[0013] An object of the invention is to provide a method of joiningfirst and second adjacent layers of a glass preform such that theinterface therebetween is substantially seed-free. A further object isto provide an improved method of joining a core region to an adjacentregion in a glass preform. Another object is to provide a method ofmaking a rod-in-tube preform by the step of cleaning the adjacentsurfaces of the rod and tube in such a manner that the outer surface ofthe tube does not become contaminated with water. Yet another object isto provide a method of forming a seed-free interface between a rod andtube in an optical fiber preform without removing glass from theadjacent surfaces of the rod and tube. Yet another object is to providea method of making fluorine-doped silica glass having high negativedelta by the OVD technique.

[0014] The present invention relates to a method of making a glassarticle. The method comprises inserting a non-porous glass core rod intoa non-porous glass tube to form an assembly that is inserted into afurnace. While the entire assembly is being heated, a centerlinechlorine-containing gas is flowed into the first end of the tube andbetween the tube and the rod, and out of the second end of the tube.Thereafter, the tube is collapsed onto the rod to form an assembly whichcan be formed into the glass article such as an optical fiber. The tubecollapse step can be performed in the same furnace in which the chlorinecleaning step occurs.

[0015] As the adjacent surfaces of the rod and tube are cleaned by thecenterline gas while the assembly is in a furnace, the outer surface ofthe tube is not contaminated by water that would be present if a flamewere employed for heating the assembly during the cleaning step.

[0016] This method is especially suitable for forming an optical fiberhaving a core that includes an annular region of depressed refractiveindex.

[0017] The tube can be formed of silica doped with fluorine or boron,both of which can be added to silica to lower its refractive index.Fluorine is the preferred dopant since attenuation due to B₂O₃ limitsfiber usage to wavelengths less than about 1200 nm.

[0018] To provide a tube doped with fluorine, a fluorine-containing gasis flowed into the aperture and outwardly through the pores of a porous,cylindrically-shaped glass preform. The porous glass preform is heatedto sinter it into a non-porous fluorine-doped tube.

[0019] A further aspect of the invention concerns a method of making aglass article having an annular region containing a high content offluorine. A tubular porous glass preform is initially formed. Thepreform is heated; and a centerline gas is flowed into the longitudinalaperture of the preform and outwardly through its pores. The centerlinegas consists entirely of a fluorine-containing compound, whereby a highconcentration of fluorine becomes incorporated in the pores of thepreform. The porous preform is heated to sinter it into a non-porousfluorine-containing glass tube. A cylindrically-shaped core rod isinserted into the fluorine-doped tube. The tube is then shrunk onto thecore rod, and the interface between the core preform and the tube isfused. An article such as an optical fiber can be formed from theresultant preform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 illustrates the formation of a porous glass preform on amandrel.

[0021]FIG. 2 illustrates the sintering of a porous glass preform.

[0022]FIG. 3 illustrates the application of a coating of glass particlesto a fluorine-doped glass tube.

[0023]FIG. 4 as a cross-sectional view of an apparatus for consolidatingand fusing the assembly formed by the method of FIG. 3.

[0024]FIG. 5 is a cross-sectional view taken along lines 5-5 of FIG. 4.

[0025]FIG. 6 is a cross-sectional view of the fused assembly resultingfrom the sintering/fusion step illustrated in FIG. 4.

[0026]FIGS. 7 and 8 are exemplary of the refractive index profiles ofoptical fibers that can be produced by the method of this invention.

[0027]FIG. 9 is a cross-sectional view of a draw furnace in which a tubeis stretched and collapsed onto a rod.

[0028]FIG. 10 is a cross-sectional view illustrating the closing of tube36.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] The method of this invention can be employed to produce anoptical fiber preform having at least one annular region containing arefractive index decreasing dopant. Basically, this method comprises (a)making a solid, non-porous glass tube containing a refractive indexdecreasing dopant throughout its entire radius, (b) inserting a solid,non-porous core glass rod into the tube, (c) cleaning the adjacentsurfaces of the rod and tube by flowing a gas containing at least 50volume percent chlorine between the rod and tube at an elevatedtemperature of at no more than 1600° C., (d) collapsing the tube ontothe rod, and (e) adding to the resultant structure a sufficient amountof cladding to form a glass article from which an optical fiber isdrawn. The core of the resultant fiber includes the inner core regionand the depressed index region and optionally includes other adjacentannular regions.

[0030] Steps (a) through (e) are not necessarily performed in the statedorder. In one embodiment, the tube is overclad with a soot coating, andsteps (c) and (d) are performed in the same furnace, the overcladpreform initially being subjected to a temperature sufficient to achievethe chlorine cleaning, the temperature then being increased to sinterthe soot and collapse and fuse the tube to the rod.

[0031] Fiber attenuation is low as a result of the low seed count at theinterface between the inner core and the depressed index regionresulting from step (c). Fiber attenuation at the water peak of about1380 nm is low since the tube is not heated by a flame in steps (c) and(d). Fibers produced by the method of this invention exhibit about 1dB/km excess loss at 1380 nm. The Rayleigh scattering loss at 1380 nmdepends on the core/clad delta. If, for example, a fiber has a Rayleighscattering loss of about 0.4-0.5 dB/km at 1380 nm; its loss is about 1.5dB/km at 1380 nm after the water peak is added.

[0032] In one embodiment of the invention, the annular preform region ofdepressed refractive index is doped with fluorine. FIGS. 1 and 2illustrate a method of making a fluorine-doped glass tube. Mandrel 10 isinserted through tubular handle 11. Mandrel 10 has a relatively largediameter in order to produce a tube having a sufficiently large innerdiameter to be useful in later steps of the method. While mandrel 10rotates, it also undergoes translational motion with respect to sootgenerating burner 13, whereby a porous glass preform 12 is built up onthe mandrel.

[0033] A standard ball joint handle 14 (see handle 44 of FIG. 3 forgreater detail) is fused to handle 11, and preform 12 is suspended inconsolidation furnace 15 by that handle. Sintering is performed in anatmosphere that includes a fluorine-containing centerline gas such asSiF₄, CF₄, C₂F₆, or the like. SiF₄ tends to give higher levels offluorine doping (typically producing a −0.7%Δ and occasionally producinga delta of about −0.8%), but that dopant causes elevated water levels inthe resultant glass. Such elevated water levels in thefluorine-containing glass can be tolerated if the fiber core has arelatively high Δ-value with respect to the silica cladding, wherebylittle power propagates in the annular fluorine-containing region of thefiber. CF₄ results in dryer glass but does not give the high dopantlevels that can be obtained by using SiF₄. High concentrations offluorine can be used in this process because porous soot preform 12 isformed of pure silica, i.e. there is no dopant such as germania thatcould be disadvantageously diffused within the blank. The resultantsinterd tube contains a relatively high fluorine concentration sincefluorine-containing gas is flowed into the tube aperture 18 (arrow 16)and outwardly through the pores of the porous glass preform whereby itachieves maximum contact with the entire body of porous glass, and sincethe centerline gas can consist of a pure gaseous fluorine compound thatcontains no diluent such as helium, chlorine or the like. Also, the onlydopant introduced into the porous preform by the centerline flow isfluorine. The end of the porous preform that sinters first preferablycontains a capillary tube 19 to prevent the muffle gases from enteringthe preform aperture and to cause most of the centerline gas to flowoutwardly through the preform interstices. A fluorine-containing gasalso flows through furnace muffle 15, as indicated by arrows 17. Whereasthe muffle gas 17 preferably contains a diluent gas such as helium and asufficient amount of chlorine to dry the preform, the centerflow gas 16preferably consists solely of the gaseous fluorine compound. However,the centerflow gas 16 could also contain one or more diluent gases suchas helium and chlorine. The flow of chlorine can be discontinued afterthe desired water content has been achieved and before the porouspreform sinters. Tube 19 is severed from the resultant fluorine-dopedtube. The resultant fluorine-doped tube can be stretched or redrawn todecrease the inside diameter to the desired size. If the tube isstretched, it can then be cut to lengths suitable for the deposition ofsoot thereon.

[0034] A boron-doped tube 27 is simpler to make than a fluorine-dopedtube. For example, a porous SiO₂—B₂O₃ preform could be formed on amandrel as described in conjunction with FIG. 1, BCl₃ being fed to theburner along with SiCl₄. The mandrel is removed, leaving a longitudinalaperture, and the preform is placed into a consolidation furnace. Amuffle gas of 40 standard liters per minute (slpm) helium flows upwardlythrough the furnace muffle, and centerline gases of 1 slpm helium and 75standard cubic centimeters per minute (sccm) chlorine flows into theaperture. After the preform is dried, it is sintered. The resultant tubecan be stretched as described above.

[0035] As shown in FIG. 3, a standard ground joint handle 44 (see FIG. 4for greater detail) is fused to one end of a length 27 of thefluorine-doped or boron-doped tube 27. A short length of silica tube 36is preferably fused to the opposite end of tube 27. Tube 27 is thenmounted in a lathe where it is rotated and translated with respect tosoot generating burner 13. Particles of glass soot are deposited on tube27 to build up coating 28. Silica tubing 36 is employed for the purposeof reducing fluorine tubing waste that would have been caused by theinability to deposit soot coating 28 on the end of tube 27 if it hadbeen secured by the lathe chuck.

[0036] Coating 28 extends over that portion of handle 44 adjacent tube27 for the following reason. During the subsequent sintering process,when that portion of tube 27 adjacent handle 44 is subjected tosintering temperature, its viscosity becomes sufficiently low that, ifthat portion of tube 27 were uncoated, it could not support the weightof the soot coated tube, i.e. the structure would drop into theconsolidation furnace. However, since the soot extends over the adjacentpart of handle 44, the entire end of tube 27 adjacent handle 44 iscovered. Therefore, the silica soot forms a sufficiently strong layerover tube 27 to support the structure during the sintering process.

[0037] Whereas a single coating 28 is shown, a plurality of sootcoatings could be deposited, the refractive index of each coatingdepending upon the desired refractive index profile of the resultantoptical fiber. To form the refractive index profile between radii r₁ andr₂ of FIG. 7, soot coating 28 could consist of pure SiO₂. To form theprofile between radii r₁ and r₃ of FIG. 8, a first soot coating ofGeO₂-doped SiO₂ could be deposited on tube 27 followed by a second sootcoating comprising pure SiO₂.

[0038] Referring to FIG. 4, the soot-coated tube is removed from thelathe, and a solid glass core rod 22 is inserted through handle 44 andinto tube 27 to form assembly 32. Rod 22 cannot fall beyond tube 36since that tube has a relatively small bore. If tube 36 were notemployed, tube 27 could be heated and tapered inwardly to form a regionof small enough inner diameter to retain rod 22. Alternatively, a smalldeformation or enlargement could be made to the top end of rod 22 tocause it to be retained by the top of tube 27. Rod 22 is preferablyformed of a glass having a refractive index greater than that of tube27, e.g. pure silica or silica doped with GeO₂, P₂O₅ or the like. Rod 22can be formed by any one of various known techniques such as modifiedchemical vapor deposition (MCVD), vapor axial deposition (VAD) andoutside vapor deposition (OVD), depending upon its desired refractiveindex profile. Two of the profiles that can be produced by the OVDtechnique are the central regions within radius r₁ of FIGS. 7 and 8. Thecentral region of FIG. 7 is a radially decreasing one while that of FIG.8 is a substantially step-profile. To make optical fibers having varioustypes of optical characteristics, such as a specific dispersion modifiedcharacteristic, the central portion of the fiber may have a differentrefractive index profile such as parabolic gradient or the like. Anyadditional layers of radius greater than that of the fluorine-doped tubealso affect optical properties such as dispersion.

[0039] Handle 44 is suspended from a support tube 46 for insertion intoconsolidation furnace 15. Handle 44 comprises glass tube 45 having aflared joint 48 at its upper end and an annular enlargement 49 spacedfrom the joint 48. Support tube 46 has a slotted handle formed in theend thereof. One side of end region 47 of tube 46 is removed to acceptthe upper end of handle 44, enlargement 49 resting on slotted base 50 asthe adjacent section of tube 45 is inserted into slot 51. At the end ofgas conducting tube 53 is a ball joint 52 which fits into cavity 54 ofjoint 48.

[0040] While assembly 32 is heated in consolidation furnace 15, a dryinggas flows upwardly through the furnace (arrows 33). The drying gasconventionally comprises a mixture of chlorine and an inert gas such ashelium. A chlorine-containing gas stream (arrow 55) is flowed from tube53 into tube 27. Although gas stream 55 could contain a diluent such ashelium, 100% chlorine is preferred for cleaning purposes. The gasstreams consist of dry gases, whereby no water is present in thevicinity of assembly 32 during heat treatment. Gases can be purchaseddry; moreover, the helium used for the muffle gas is also run through adrier.

[0041] Since the diameter of rod 22 is slightly smaller than the innerdiameter of tube 27, the chlorine flows downwardly around the entireperiphery of rod 22; it exhausts through tube 36. To facilitate the flowof chlorine past the bottom end of rod 22, that end can be provided withone or more slots 23 at the periphery of the bottom surface (FIGS. 4 and5). The chlorine acts a hot chemical cleaning agent. During this hotchlorine cleaning step, the temperature is below the sinteringtemperature of soot coating 28 so that the space between rod 22 and tube27 remains open for a sufficient length of time for the requiredcleaning to occur. The chlorine cleaning step is more effective at hightemperatures. It is preferred that the temperature of the cleaning stepbe at least 1000° C., since at lower temperatures, the duration of thestep would be sufficiently long that the step would be undesirable forcommercial purposes. Obviously, lower temperatures could be employed ifprocessing time were not a concern. The temperature should not be over1600° C. for reasons given above and is preferably no more than 1500° C.The flow of hot chlorine between the fluorine tube and rod 22 is verybeneficial in that it allows the surfaces of the two members to bebrought together without the formation of seeds at their interface.Seeds include defects such as bubbles and impurities that can produceattenuation in the resultant optical fiber. The centerline gas flow 55continues until tube 36 begins to collapse as shown in FIG. 10.

[0042] As soot coating 28 sinters, it exerts a force radially inwardlyon tube 27, thereby forcing that tube inwardly against rod 22 to form afused assembly 38 (see FIG. 6) in which the three regions 22, 27 and 28′are completely fused. A relatively low density soot provides a greaterinwardly directed force; however, the soot coating must be sufficientlydense to prevent cracking.

[0043] It was previously indicated that tube 36 need not be used,whereby other means would be employed for holding rod 22 in tube 27. Forexample, rod 22 could be suspended by an enlarged end as shown in FIG.9, or the bottom end of tube 27 could be subjected to a heat treatmentand its diameter made sufficiently small to secure rod 22. If tube 22were not present, the sintering of soot coating 28 would cause thebottom end of tube 27 to collapse onto rod 27 and prevent further flowof centerline gas 55.

[0044] Fused assembly 38 can be drawn directly into an optical fiber inwhich layer 28′ forms the outer region. Alternatively, fused assembly 38can be provided with additional cladding prior to drawing an opticalfiber. For example, an additional coating of cladding soot can bedeposited onto assembly 38 in the manner shown in FIGS. 1 and 3; theadditional coating can be dried and sintered, and the resultant preformcan be drawn into an optical fiber.

[0045] In accordance with another aspect of the invention soot coating28 is not deposited on tube 27, and tube 27 is not collapsed onto rod 22in furnace 15. The assembly including rod 22, tube 27, tube 36 and balljoint handle 44 is subjected to an elevated temperature in a furnacewhile chlorine flows between rod 22 and tube 27 as discussed above. Thetemperature preferably remains within the range of about 1000° C. to1500° C. to chemically clean the surfaces of members 22 and 27. After asufficient period of time has elapsed to permit chemical cleaning tooccur, the cleaned assembly 63 is removed from that furnace and isinserted into a conventional draw furnace (FIG. 9). The top end of rod22 is provided with an enlarged end 65 which is suspended from a narrowregion at or near handle 44. In the illustrated embodiment, the insidediameter of the bottom end of handle 44 is larger than the insidediameter of the top end of tube 27; this provides a ledge for supportingenlargement 65. A source of vacuum (not shown) is connected to handle44. The bottom tip of assembly 63 is heated by resistance heater 62. Asthe tip of assembly 63 passes through heater 62, the diameter of theassembly decreases, and tube 27 collapses onto rod 22 and the spacebetween those two members becomes evacuated. Further drawing of assembly63 causes the assembly to elongate into a core preform rod 66 in whichtube 27 is fused to rod 22. The core preform rod is severed intosuitable lengths which are provided with cladding and drawn into opticalfiber as described above.

[0046] Typical step-index optical fibers that were designed for use atwavelengths around 1300 nm exhibit a positive dispersion in the 1550 nmwindow where the fiber exhibits lowest attenuation. Such a system can beupgraded for operation in the 1550 nm window by placing in series withthe step-index fiber a dispersion compensating (DC) fiber having arelatively high value of negative dispersion at 1550 nm. The followingexample describes the manufacture of such a DC fiber.

[0047] A single-mode DC optical fiber having the refractive indexprofile illustrated in FIG. 7 was made as follows. A 0.25 inch (0.64 mm)alumina rod was inserted through the center an alumina tube having a 1.5inch (3.8 cm) outside diameter. Rubber corks were used at the ends ofthe alumina tube to center the alumina rod within it. Handle 11 wasplaced near one end of the alumina tube. Pure silica soot was depositedon the alumina tube and on a portion of the handle. A detaileddescription of a method of forming a porous preform on an alumina tubecan be found in U.S. Pat. No. 5,180,410.

[0048] A standard ball joint handle 14 was fused to the silica handle 11prior to consolidation. Consolidation was carried out in the mannerdescribed in conjunction with FIG. 2. The centerflow gas 16 consisted of1.5 slpm SiF₄. Muffle gas 17 consisted of 20 slpm He, 0.5 slpm Cl₂ and1.0 slpm SiF₄.

[0049] The sintered fluorine-doped tube contained about 2.4 wt. %fluorine (the Δ-value of the tube with respect to silica was about −0.7%Δ). The tube was redrawn to form an elongated tube having an outsidediameter of approximately 12 mm and an inside diameter of 6.1 mm. A 30inch (76 cm) long piece of fluorine-doped tubing 27 was severed from thesintered tube. A standard ground joint handle 44 was fused to a firstend of tube 27. A 4 inch (10 cm) long silica tube 36 having inside andoutside diameters of about 3 mm and 12 mm was fused to the second end oftube 27. The ends of the resultant tubular structure were mounted in alathe where it was rotated and translated with respect to flamehydrolysis burner 13 (FIG. 3). Particles of SiO₂ soot entrained in theburner flame were deposited on tube 27 to build up a coating 28 having alength of 70 cm and a outside diameter of 90 mm. Coating 28 extendedover the entire length of tube 27, and it extended a longitudinaldistance of about 50 mm along handle 44. The coated structure 30 wasthen removed from the lathe.

[0050] The following method was used to make core rod 22. The largediameter end of an alumina mandrel was inserted into a glass tubularhandle. The outside diameter of the mandrel tapered from 5.5 mm to 6.5mm over its 107 cm length. The ends of the mandrel were mounted in alathe where it was rotated and translated. GeO₂-doped SiO₂ soot wasdeposited on the mandrel and a portion of the handle. The reactantsGeCl₄ and SiCl₄ were initially flowed to the burner in sufficientquantities to form soot formed of SiO₂ doped with 37 wt. % GeO₂. Witheach pass of the burner with respect to the mandrel, the flow of GeCl₄was decreased, the last pass depositing pure silica soot. The flow ofGeCl₄ to the burner decreased in accordance with such a recipe that theradial decrease in the concentration of GeO₂ in the resultant fiber wassubstantially parabolic.

[0051] After the deposition of a soot preform to a thickness of 100 mm,the mandrel was removed by pulling it out through the handle, therebyleaving a longitudinal aperture. A capillary tube was inserted into theend of the porous preform aperture opposite the handle. The porouspreform was suspended in a consolidation furnace, and a centerlinedrying gas comprising 1.0 slpm helium and 50 sccm chlorine was flowedthrough the handle, into the preform aperture, and outwardly through thepreform interstices. A muffle gas comprising 40 slpm helium flowedupwardly through the furnace. The maximum temperature of theconsolidation furnace was 1460° C. The aperture of the capillary tubeplug closed during the sintering process.

[0052] The sintered preform was inserted into a draw apparatus where itstip was heated to 2100° C. while a vacuum connection was affixed to itsupper end in the manner disclosed in U.S. Pat. No. 4,486,212, which isincorporated herein by reference. After the end of the preform wasstretched so that its aperture was either very narrow or completelyclosed, the aperture was evacuated. As the lower end of the preform waspulled downwardly at a rate of about 15 cm/min, and its diameterdecreased, the evacuated aperture collapsed. The diameter of theresultant rod was approximately 6 mm. The refractive index profile ofthe resultant stretched rod was similar to that between the axis andradius and r₁ of FIG. 7. A rod 22 having a length of 70 cm was severedfrom the stretched rod. Two slots 23 were sawed at the periphery of thatend 24 of rod 22 that was to form the lower end in the subsequentconsolidation process.

[0053] Rod 22 was inserted through handle 44 and into fluorine-dopedtube 27 until end 24 thereof contacted tube 36, thereby forming thesoot-coated assembly 32 of FIG. 4. Handle 44 of assembly 32 wassuspended from a support tube 46 for insertion into the consolidationfurnace. While assembly 32 was rotated at 1 rpm, it was lowered intoconsolidation furnace muffle 15 at a rate of 5 mm per minute. A gasmixture comprising 50 sccm chlorine and 40 slpm helium flowed upwardlythrough the muffle. The centerline gas flow 55 consisted of 0.5 slpmchlorine. The chlorine flowed downwardly around rod 22 and exhaustedthrough tube 36. The maximum temperature in the consolidation furnacewas 1500° C. As assembly 32 moved downwardly into the furnace, thetemperature of assembly became high enough that the centerline chlorineflow cleaned the adjacent surfaces of rod 22 and tube 27. As assembly 32moved further into the furnace, first its tip and then the remainder ofthe assembly was subjected to the 1460° C. temperature which wassufficient to sinter coating 28. During sintering of soot coating 28,tube 27 was forced inwardly against section 22, and the contactingsurfaces became fused, thereby forming fused assembly 38.

[0054] Assembly 38 was removed from the consolidation furnace and wasinserted into a draw furnace. The lower end of the preform was heated toabout 2100° C., and it was drawn to form a rod having a diameter of 5.5mm.

[0055] A 90 cm section was severed from the resultant rod, and it wassupported in a lathe where it functioned as a mandrel for the depositionof an additional coating of cladding glass soot. Deposition wascontinued in the manner described in conjunction with FIG. 1 until alayer of SiO₂ particles having an outside diameter of 100 mm wasdeposited to form a composite preform.

[0056] The resultant composite preform was gradually inserted into aconsolidation furnace having a maximum temperature of 1450° where it wassintered while a mixture of 99.5 volume percent helium and 0.5 volumepercent chlorine flowed upwardly through the furnace muffle. Theresultant sintered draw blank, the diameter of which was about 50 mm,was inserted into a draw furnace where the tip thereof was subjected toa temperature of about 2100° C. The draw blank was drawn to form adispersion compensating optical fiber having an outside diameter of 125μm. The single-mode cutoff value of the fiber was 750 nm. At awavelength of 1550 nm, the attenuation was 0.5 dB/km and dispersion wasmore negative than −90 psec/km nm. The lowest value of dispersion forfibers made by this method was −105 psec/km nm.

[0057] Prior to the present invention, seeds formed at the interfacebetween the fluorine tube and germania rod when those two members werebrought together. This process essentially completely eliminates seedsas evidenced by the fact that blanks yielding 50 km of fiber wereconsistently drawn with no upsets, i.e. fiber attenuation at 1550 nm wasconsistently around 0.5 dB/km.

1. A method of making a fiber comprising: making a fluorine doped glasstube via a process, which involves: forming a tubular porous glasspreform having an aperture therethrough, placing the porous glasspreform in a furnace and heating the preform while injecting afluorine-containing gas into the furnace to form a fluorine doped poroustube, and heating the fluorine doped tube to sufficiently consolidatethe tube into a glass fluorine doped tube, positioning a glass rodwithin the fluorine doped glass rod; and collapsing and sintering thetube onto the rod to form a core glass preform.
 2. The method of claim1, further comprising prior to the collapsing and sintering step,depositing a layer of glass particles on said tube to form a soot coatedassembly, and said collapsing and sintering step comprises heating thesoot coated assembly to a temperature sufficient to sinter said coatingthereby generating a radially-inwardly directed force that facilitatessaid tube collapsing onto and fusing to said rod.
 3. The method of claim1, further comprising prior to the collapsing and sintering step,depositing a layer of glass particles on said tube to form a soot coatedassembly, and said collapsing and sintering step comprises heating thesoot coated assembly to a temperature sufficient to sinter said coatingthereby generating a radially-inwardly directed force that facilitatessaid tube collapsing onto and fusing to said rod.
 4. The method of claim1, further comprising, prior to said collapsing step, flowing acenterline gas through said tube, said centerline gas comprised of atleast 50% chlorine.
 5. The method of claim 1, further comprising, priorto said positioning step, redrawing said glass tube to have an internaldiameter which is narrower than the internal diameter of said tube priorto said redrawing step.
 6. The method of claim 1, wherein said fluorinedoping step comprises doping the tube with an amount of fluorine whichis sufficient to give said tube a Δ-value of less than −0.5% withrespect to silica, where Δ_(a−b)=(n_(a) ²−n_(b) ²)/(2n_(a) ²), n_(a)being the refractive index of the fluorine-doped glass and n_(b) beingthe refractive index of silica.